The present invention relates in general to an improvement of an unlimited sliding ball spline assembly, and, in particular, to the ball spline assembly capable of transmitting a large torque and obtaining a ball spline assembly combination without any backlash.
Referring to FIG. 1, there is shown a conventional design of the unlimited sliding ball spline assembly. Such ball spline assembly comprises a thick cylindrical outer sleeve 2, the outer sleeve 2 being machined on the inside wall surface thereof to form plural parallel sleeve grooves 4 and further being machined to drill plural return holes 5 for non-loaded balls 6, the sleeve grooves 4 and return holes 5 extending substantially the entire length of the outer sleeve 2, a shaft 1 being inserted in the outer sleeve 2, the shaft 1 being machined on the outside wall surface thereof to form plural parallel shaft grooves 3 which extend substantially the entire length of the shaft 1, the parallel sleeve and shaft grooves 3 and 4 being opposite each other and forming plural rectilinear raceways for loaded balls 6, the return holes 5 and the rectilinear raceways for loaded balls being connected by connecting grooves to form endless raceways for balls 6, and relative displacements in the rectilinear directions and transmission of torque between the outer sleeve 2 and the shaft 1 being carried through the loaded balls 6 in the endless raceways.
However, in such conventional ball spline assembly, the diameter of the shaft 1 must be smaller than the minor diameter of the outer sleeve 2. Accordingly, when the shaft 1 rotates in the direction of arrow A of FIG. 1, the bearing surface 7 in each shaft groove 3 of the shaft 1 which contacts with the loaded balls 6 and receives the contact pressure in the direction of rotation of the shaft 1 becomes the arc a.sub.1 b.sub.1 which is formed from the intersecting point a.sub.1, which the imaginary line between the center of the loaded ball 6 and the center axis of the shaft 1 crosses with the shaft groove 3, to the end point b.sub.1 of the shaft groove 3, and this arc a.sub.1 b.sub.1 becomes inevitably smaller than the quarter circumference of the ball 6, and, further, an angle .theta..sub.1, which the total contact pressure P in the perpendicular direction against the arc a.sub.1 b.sub.1 take against the imaginary line between the center of the loaded ball 6 and the center axis of the shaft 1, becomes inevitably smaller than 45.degree.. Therefore, it is unavoidable that the component P.sub.1 of the total contact pressure P in the direction of rotation becomes small and it is not possible to transmit a large torque.